Overvoltage protector for a device having two parts movable relatively to each other

ABSTRACT

An overvoltage protector in an apparatus having two moving parts (1,5). The parts are connected via a bearing (2,3,4), one part being stationary in relation to a substructure (13) at an electrical ground potential. The moving part (5) can be subjected to a voltage and the stationary part (1) is electrically connected to the grounded substructure. For preventing current from passing through the bearing between the parts, an electrical insulator (6,7) is provided in the current path which can occur through the bearing as a result of different potentials of the parts. The moving part (5) has a ring (9) spaced from a ring (8) connected to electrical ground such that a spark discharge gap (10) is formed between the parts. The bearings (2,3,4) are protected against current flow by the electrical insulator (6,7) up to a given voltage threshold. In order that this voltage threshold will not be exceeded, the spark discharge gap (10) comes into operation at a lower value than this voltage threshold and keeps the voltage difference between the parts at this lower value.

FIELD OF THE INVENTION

The invention relates to an overvoltage proctector for a device havingtwo parts movable relative to each other, one of which is stationary inrelation to a frame at ground potential, said parts being connected witheach other via at least one bearing, the movable part being subjected toan electrical voltage and the stationary part being electricallyconnected to the frame.

BACKGROUND

In apparatus with mutually moving parts which are connected to eachother via a bearing and are subjected to different electricalpotentials, there is the risk that a current path can occur through thebearing. The current passing the bearing surfaces may be of magnitudethat the bearing is damaged by pitting of the bearing surfaces. It isknown to protect the bearings by insulating the bearing seatings in thecases where the voltage difference is small, as is the case, forexample, for asynchronous machines. This is not satisfactory when thereare large voltage differences, since there is the risk of flashover.Examples of this are to be found in wind turbines and radar antennaewhich are subjected to lightning.

SUMMARY OF THE INVENTION

The problems mentioned above are solved by providing the bearing withinsulation while the moving part is provided with a spark discharge gapat the same time.

The bearing is protected by the insulation against electric current upto a given voltage threshold. In order that this voltage threshold willnot be exceeded, the spark discharge gap comes into operation and lowersthe voltage.

BRIEF DESCRIPTION OF THE DRAWING

The sole FIGURE of the drawing shows, partly in section, an elevationalview of one embodiment of apparatus according to the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The invention will now be described in detail with reference to theappended drawing. Therein is seen a bearing housing 1 with ball bearings2, 3 and 4, which carry a shaft 5, for example, the shaft of a windturbine 15. The shaft is insulated from the ball bearings by bearingseatings 6 and 7, consisting of material with high electrical insulationpower. The housing is connected to electrical ground potential by aframe 13, and carries at one end a ring 8 having a bevelled outer edge.The shaft supports a ring 9 coacting with, and surrounding the ring 8,and having an inner surface with a bevel corresponding to that of theouter edge of the ring 8, such that a circular gap 10 is formed betweenthe two rings. This gap constitutes an electrical flashover zone duringa spark discharge. The shaft 11 of a machine 14 such as a generator,which is driven by the shaft 5, is connected to the shaft via aninsulating part 12. The thickness and material of the bering seatings 6and 7, which determine the dielectric strength, and the axial extensionof said seatings, which determines the flashover resistance, are such inrelation to the insulation power of the gap 10 that the flashovervoltage is lower across the gap than the dielectrical breakdown voltageand the flashover voltage over the bearing seatings. If the shaft issubjected to a voltage lower than the flashover voltage, the insulatingbearing seatings prevent current from passing through the bearings. Fora voltage exceeding the flashover voltage of the air gap, e.g.lightning, a spark discharge is produced in the gap 10 which limits thevoltage which can occur at the bearing seatings.

As an alternative to the described embodiment, instead of insulatingbearing seatings, the shaft may be provided with an insulating layer orconsist entirely of insulating material at the portion in contact withthe bearings. If the shaft is mounted in journal bearings, the bushesthemselves may consist of insulating material. The arrangement inaccordance with the invention thus protects the bearings from conductingcurrent and thus being destroyed. Bearing damage in a wind turbine orradar antenna results in great inconvenience, since the entire machinemust be taken down from its tower and transported to a workshop forrepairs. Other inconveniences are also avoided. The gap 10 works withoutfriction and is only worn during spark discharges. These parts caneasily be dimensioned for long life and can be made easily exchangeable.

The invention is not limited to the described embodiment and can also beapplied to bearings for linear movement, e.g. where a bearing carries aslide on a path along which the slide can move. The bearing iselectrically insulated from the slide. The zone for spark dischargeconsists in this case of a projection on the slide and an electricallyconductive rail running alongside the path at a distance from theprojection.

What is claimed is:
 1. Overvoltage protected apparatus comprising afirst stationary member at ground potential, a second movable membersubject to electrical voltage, bearing means between said members toprovide relative movement therebetween, and overvoltage protection meansfor opposing current flow through said bearing means between saidmembers, said overvoltage protection means comprising electricalinsulative means between said bearing means and said second member toprevent current flow between said members due to voltage differencebetween said members thereby to protect said bearing means from damagedue to current flow therethrough, and means on said members providing aspark gap therebetween which remains substantially constant duringrelative movement of said members, the combination of said electricalinsulation means and said spark gap preventing current flow through thebearing means due to voltage difference between said members resultingfrom movement of the second movable member and the grounded stationarymember as well as due to application of external voltage to said movablemember.
 2. Apparatus as claimed in claim 1 wherein said electricalinsulation means between said bearing means and said second movablemember has a dielectrical breakdown voltage and flashover voltage whichis greater than the flashover voltage at said gap.
 3. Apparatus asclaimed in claim 2 wherein said electrical insulation means compriseselectrically insulating bearing seats for said bearing means, said seatsbeing mounted on said movable member.
 4. Apparatus as claimed in claim 2wherein said means providing the spark gap comprises first and secondrings respectively on said first and second members defining said sparkgap therebetween.
 5. Apparatus as claimed in claim 4 wherein said ringsextend radially from said members to form said gap around said bearingmeans at a radial distance therefrom.